1. Field of the Invention
The present invention relates generally to lithography systems. More particularly, the present invention relates to sensors used for focusing in lithography systems.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a reticle is used to transfer a desired pattern onto a substrate, such as a wafer. Reticles may be formed of material(s) transparent to the lithographic wavelength used, for example glass in the case of visible light. In addition, reticles can also be formed of material(s) that reflect the lithographic wavelength chosen for the specific system in which it is used. An illumination source (e.g., exposure optics located within a lithographic apparatus) illuminates the reticle, which is disposed on a reticle stage. This illumination exposes an image onto the substrate that is disposed on a substrate stage. The image exposed onto the substrate corresponds to the image printed on the reticle. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the substrate, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the substrate.
Step and scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire substrate at one time, individual fields are scanned onto the substrate one at a time. This is done by moving the substrate and reticle simultaneously, albeit at different rates, such that the imaging slot is moved across the field during the scan. The substrate stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the substrate surface. In this manner, the quality of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image quality, image degradations generally occur in such systems due to imperfections, such as those within the projection optics system, illumination system, and the particular reticle being used.
An important aspect in lithography is maintaining uniformity in the size of the features created on the surface of substrates. Current requirements for variations in feature size (also known as critical dimension) are that they be less than approximately ±5% of nominal. This implies, for example, ±5 nm or less variations in critical dimension for 100 nm isolated lines, ±3.5 nm or less variations for 70 nm isolated lines, and ±1.5 nm for 30 nm isolated lines. Critical to achieving these levels of performance is the focus system.
Some current state of the art focus systems use capacitance gauge sensors. Focus sensor capacitance gauge metrology is based on the change in capacity of a plane parallel plate capacitor when the plate separation is changed. The capacity of a plane parallel plate capacitor with a small plate separation with respect to the plate diameter that is filled with uniform dielectric layers over a conductive substrate is inversely proportional to the plate separation. Such current state of the art focus systems can meet current lithography needs. However, the reduction of both random and systematic errors is required for next generation lithography use. Current random errors are of the order of 2 nm-rms (root mean squared) over the servo bandwidth. Current mean shifts, not related to wafer processing, are of the order of 25 nm. Current systematic errors can exceed 100 nm. Current wafer process errors can range from being negligible to over 100 nm.
Current understanding of these errors attributes the random errors to the capacitance gauge electronics. Mean shifts are attributed to systematic changes in the thickness or properties of layers underlying the resist. The wafer process errors are attributed to the wafer circuit layers and patterns. For a given integrated circuit, the wafer process errors are a systematic offset that is nominally the same for each field. Capacitance focus sensor readings are a function of the dielectric layers overlaying the conductors and the special distribution of the patterns. For example, a capacitance sensor will indicate a distance change when the wafer target changes from a large area dielectric film to a conductive film both on the same surface and of the same thickness. In addition, a capacitance gauge will give readings that are a function of conductor feature size for the same average conductor fill factor.
Accordingly, there is a need for improving focus systems so that they meet the requirements for next generation lithography use. More specifically, there is a need for improved focus systems that can be used to maintain uniformity in size of features created on the surface of substrates.